We have recently shown that pulsed infrared radiation (IR) applied to the semicircular canal crista ampullaris in vivo evokes dramatic and highly controllable responses of first order afferent neurons. Many afferents phase lock to the stimulus and fire an action potential for each IR pulse. This allows IR stimuli to be used to control the timing and rate of action potentials transmitted by the 8th nerve to the brain. The cellular response is robust, even under long-term stimulation. Our in vitro data using cardiomyocytes indicates that IR evokes mitochondrial Ca2+ flux via the uniporter and the Na+/Ca2+ exchanger. It is not know if mitochondrial IR excitability also underlies the exquisite sensitivity of hair-cell sensory epithelia to IR, or if some other mechanism is at play. The mechanism of IR action will be determined through examination of 4 specific aims. We will 1) establish dose-response functions for pulsed IR and heat across a broad parameter space, 2) quantify cellular electrophysiological and [Ca2+] responses to IR, 3) record postsynaptic potentials and currents in afferent nerve endings evoked by IR applied to hair cells, and 4) examine the mitochondrial component of IR excitability. Results are expected to determine the fundamental mechanism of IR photosensitivity in hair cells. Knowing the mechanism will have immediate impact on how pulsed IR is used for basic science (e.g. to evoke mitochondrial Ca2+ transients vs. membrane depolarizing currents). Results also have potential for significant impact on the development of new IR therapeutic interventions and prosthetic devices that take advantage of the mechanism of IR action.